Dephasing or phase-shifter circuit



Jan. 14, 1941. H, mEgx 2,228,869

DEPHASING OR PHASE-SHIFTER CIRCUIT Filed Feb. 21, 1959 Fig.1

/ E OUTPUT INVENTOR HENRI C IRE/X ATTORN EY Patented Jan. 14, 1941UNITED STATES DEPHASING OR PHASE-SHIFTER CIRCUIT Henri Chireix, Paris,France, assignor to 00mpagnie Generals de Telegraphic Sans Fil, acorporation of France innit-cation February 21, 1939, Serial No. 257,605In Germany May 23, 1938 2- Claims.

It is known in the prior art that the iterative impedance in theband-pass of an electric filter is a real quantity. It is for thisreason that the filter is terminated generally by a pure resistancehaving a value equal to the iterative impedance of this filter for aparticular value of the frequency contained in the band-pass. In View ofthis fact, the phase of the voltage at the output end is causedprogressively to lag with respect to the input voltage in a measure asthe frequency increases within the band-pass width. It is easy to see inthe light of the equations underlying filters that it is possible toobtain the opposite result, that is, an advancing or leading shift ofthe phase of the output voltage by choosing for the output impedance anegative resistance without altering the cut-off limits or the width ofthe bandpass filter.

Such a resistance may be formed by different ways and means basicallyknown in the art, that is, either by means of multi-electrode tubespredicated upon seccndary-emission phenomena or on negative or reversecoupling (dynatron, electrcn-oscillaticn or retarding-field tubes) or byemploying magnetrons of the split-anode type, or else by using electronmultipliers, etc. i In these conditions, however, the input impedance ofthe filter inside the band-pass also is found to be a negativeresistance. Thus, in order to avoid an unstable state, a positiveresistance (sufliciently high when in series, or sufiiciently low whenin parallel) is inserted at the input end of the filter in order thatthe total apparent resistance may be positive.

For a better understanding of the invention, reference will be had tothe accompanying drawing, wherein Fig. 1 is a schematic representationof a filter circuit embodying the present invention, Fig. 2 is a circuitof a particular embodiment of the invention, Fig. 3 is thecharacteristic curve of a negative resistance device utilized in theinvention, Fig. i is an alternative bridge circuit capable of use inFig. 2, Fig. 5 is a modified bridge circuit, and Fig. 6 is the circuitof Fig. 2 transformed to include the bridge circuit of Fig. 5.

In the circuit scheme of 1 showing a T- typc filter of ladder structureR1 is a negative resistance employed in the output of the filteraccording to the invention and R2 is a positive resistance at the inputend of the filter in order that the total apparent resistance may bepositive thereby avoiding the instability. A more particular embodimentshall now be given in more detail, namely, that of a dephaser or phaseshifter whose band-pass extends from zero frequency to infinitefrequency. Such a dephaser resulting in a phase lag at the output endmay be formed by using one or more meshes in a Wheatstone bridge whosearms are formed by pure reactances, for instance, two self inductancesand two capacities.

The band-pass of such a filter, in fact, is determined by the space orinterval in which the two groups are of opposite signs, that is, in thepresent instance from zero to infinity, and the iterative impedance byThe iterative impedance is thus constant for all frequencies and thefilter may be terminated by-a resistance R in sucha manner that R L/C,or

The output voltage is thus equal to the input voltage for allfrequencies, but undergoing a progressive backwards or lagging phaseshift as the frequency increases.

Also according to the present R L/C, whereas that is by employing anegative resistance.

In this case the output voltage is still equal to the input voltage forall frequencies, but it is dephased gradually in a leading sense whenthe frequency increases. The input impedance, besides, will be equal toand thus be negative, and a positive resistance is necessary tostabilize the whole arrangement or assembly. Thus, the circuit scheme inFig. 2 is obtained which just represents a particular or exemplifiedembodiment.

The source of current I by way of a positive resistance 2 feeds thebridge 3. Across the other diagonal of the bridge there is preferablyconnected a step-up transformer 4 which in turn is connected to twotriodes (or multi-electrode tubes) 5 symmetrically arranged. The outputis delivered to the secondary of transformer 6. The negative resistanceis simply obtained by employing the secondary emission of the grid inthe region where the grid is negative. In fact it is known in the priorart that with many triodes, characteristics i fw such as shown in Fig. 3can be obtained, that is characteristics having a negative slope betweenA and B. Thus by the aid of the source of current 1, the operating pointis chosen so as to be situated in the straight part of this region orrange A-B.

The transformer 4 is not liable to cause appreciable disturbances if itis properly calculated for the range of desired frequency band (lowleakage, low magnetizing current). Besides, it has the advantage ofadapting or matching more easily the (high) negative resistance to theinvention iterative impedance of the network, which is generally muchlower.

Naturally, there are conceivable numerous other ways and means adaptedto design circuit organizations of the same nature. For instance, asymmetric arrangement is not indispensable, although preferable; neitheris the provision of the transformer 4.

Also the negative resistance may be obtained in a different manner. Thebridge itself may be designed in a different way, for instance, in amanner as shown in Fig. 4, where two arms are formed by a resonantcircuit and the two other arms by an anti-resonant circuit, the resonantand anti-resonant frequencies coinciding. With such an organization aconstant iterative impedance is again obtainable for all frequencies anda range extending from zero frequency to infinite frequency, while thetotal rotation of the phase is doubled.

The invention is particularly advantageous when applied in amplifierswhere reverse feedback is employed in order to increase the stability ofamplification or to reduce distortion. It is known in the prior art thatthe use of negative feedback is limited by the rotation or shift of thephase introduced by the amplifier, such phase rotation causingoscillating. By introducing a dephaser of a nature such as described,causing a shift in a leading sense in the path of the negative feedback,it is possible to more or less completely compensate the dephasingcaused by the amplifier. Another modified embodiment which is moreeconomical so far as circuit material is concerned, is as follows:

A network in the form of a complete bridge containing two reactances7'21 and two reactances 722 may be replaced by a half bridge scheme,according to Fig. 5, which contains only one re- There thus results theexpression The band-pass is always defined by ZiZz 0, but the outputVoltage is more constant when R differs from Z1Z2 than in the case of acomplete network fed by a constant voltage.

On the contrary, it will be seen that the dephasing as given by theexpression changes its sign when R changes its sign, all other thingsremaining equal.

Thus everything being taken into account, Fig. 2 changes into Fig. 6where the same reference numerals designate the same elements. One ofthe input leads is connected to the mid-point of the primary oftransformer 4; also here the resistance 2 plays the part of astabilizing resistance.

If it is desired, besides, to obtain a reduction of amplitude in thelow-frequency and in the high-frequency ranges, a network may beconstructed in the form of a band-pass filter comprising reactances :iZrand 1Z2, for instance, in the form of two anti-resonant circuits tuned,respectively, to the cut-off frequencies.

What I claim is:

l. A circuit for shifting the phase of the output voltage with respectto the input voltage comprising input and output terminals, 9, seriesresistance connected to one of the input terminals, a series resonantcircuit including an inductance and a capacity connected between saidresistance and the other input terminal, a second series resonantcircuit in parallel to the first series circuit with its inductance andcapacity in reversed relation with respect to the corresponding elementsof the first circuit, a transformer having its primary connected to eachjunction point of the inductance and capacity of the respective resonantcircuits, and a negative resistance device connected between thetransformer secondary and the output terminals.

2. A circuit as defined in claim 1 wherein the negative resistancedevice comprises a pair of push-pull electron discharge tubes, the gridsof which are positively biased.

HENRI CHIREIX.

